Oscillation generator



1951 B. c. FLEMING-WILLIAMS 2,541,230

' I OSCILLATION GENERATOR Filed Feb. 24, 1948 R15 .Dl

I/VVENTOR Brian C. Fleming-Williams 31/ a. AW

Patented Feb. 13, 1951 OSCILLATION GENERATOR Brian Clififord Fleming-Williams, London, England, assignor to A. C. Cossor Limited, London, England, a British company Application February 24, 1948, Serial No. 10,387 In Great Britain January 25, 1945 Section 1, Public Law 690, August 8, 1946 Patent expires January 25, 1965 8 Claims. 1

This invention relates to thermionic valve circuits suitable for generating potential sweeps which are linear with time and/or potential waves of square-wave form.

The principal object of this invention is to provide an improved circuit for either or both of these purposes.

The accompanying drawing shows, by way of example only, a circuit diagram of an arrangement embodying the invention in preferred form.

In this arrangement a ganged pair of switches SI, S2 is provided to change over, as required, between two alternative modes of operation. With switches SI, S2 both in position I, the apparatus generates in response to the application of a firing pulse, a single linear voltage sweep followed by a rapid automatic return; and it will then be ready to repeat this performance at any time in response to a further firing pulse. At the same time, a single square-wave cycle of potential is generated at another output terminal. With the switches both in position 2, however, the apparatus operates as a self-running generator.

The principal valve VI is a pentode which may be of the VR. 116 or Mazda AC/SPI type. The cathode of pentode VI is connected to earth through a biasing resistor R5. The control grid of pentode VI is coupled to a point in the anode circuit of pentode VI by a condenser Cl, and is also connected through a high resistance R3 to the positive line, which may be at 300 volts to earth, preferably stabilized. A small stopper resistance RIO is connected in the lead to the control grid to prevent parasitic oscillations. A diode D2 having its cathode connected to earth prevents the potential of the control grid of pentode VI from rising appreciably above that of earth.

The anode of pentode VI is connected to the positive line through an anode load comprising two parallel paths. One of these paths consists of resistor R4. The other path consists of the space path of a second pentode V2, in series with resistors RI I, BIZ. The anode of pentode VI is also connected through diode DI and resistor RI3 to the slider of a potentiometer Rt, which is itself connected between the positive line and earth. This diode circuit prevents the potential of the anode of pentode VI from rising appreciably above the potential selected by the slider of potentiometer RI.

When the switch S2 is in position I, the screen and suppressor of pentode VI are coupled by a condenser C2. The screen current is derived from the positive line through resistors R6 and R1, the Voltage across resistor R6 being smoothed by condenser C4 to earth. The suppressor is biased through resistor R2 from a source which may be about 15 volts negative to earth, the switch SI being in position I.

A diode D4, connected between the suppressor and cathode of pentode VI, is provided to prevent the suppressor potential from rising appreciably above that of the cathode.

Provision is made for firing alternatively by either positive or negative pulses. Terminal T3, for the application of negative firing pulses, is coupled to the cathode of diode DI through condenser CB. Terminal TI, for the application of positive firing pulses, is coupled to the anode of diode D3 through condenser C3. The anode of diode D3 is biased from the slider of potentiometer RM, the voltage across which is smoothed by condenser C4.

The manner of operation of the circuit with both switches in position I will now be described.

The circuit has a stable condition with the anode of pentode VI approximately at the potential of the slider of potentiometer RI and with the suppressor at the potential of its bias: source: the whole of the cathode current of pentode VI is then flowing to its screen, the control grid is held approximately at earth potential by diode D2, and the potential between cathode and control grid is therefore determined by resistor B5.

In this stable condition of the circuit, cathode current in the other pentode V2 is kept down to a fraction of a milliamp. This low current sets up a voltage drop across resistor BIZ, which holds the control grid of this pentode sufficiently nega tive to limit its current. The total current through diode DI is made up of this small current together with current of comparable magnitude flowing along the parallel path through resistor R4.

Suppose that a negative pulse is now applied at terminal T3: then the potential of the anode of pentode VI will fall. So also will the potential of the control grid of pentode V2 and hence that of its cathode. Owing to the coupling through condenser CI between the cathode of pentode V2 and the control grid of pentode VI, this latter control grid is driven negative, with the result that diode D2 is cut oil and the cathode current or pentode VI is reduced. The potential of the screen or" pentode VI therefore rises; and owing to its coupling through condenser C2, it carries the suppressor potential with it. Anode current then begins to flow in pentode VI causing a further lowering of the potentials of the anode of pentode VI, and of the control grid and the 3 cathodeof pentode V2, the elfect is cumulative and proceeds extremely rapidly until a metastable state is reached in which the anode current of pentode VI is approximately the sum of the currents through resistors R3 and R3. The condenser CI is now being discharged through the anode-cathode path of pentode VI and through resistor RI2, and the current through this resistor is now more than sufiicient to cut ofi pentode V2 completely.

During the change from the stable to the metastable state, the fall of potential of the anode of pentode VI is not more than about 6 volts, while that of the cathode of pentode V2 and that of the control grid of pentode VI are each only about 3 volts. At the same time the rise of screen poten-' tial of pentode VI may be about 60 volts. Its suppressor is however, prevented by diode DE from rising appreciably above cathode potential, and the condenser C2 therefore becomes charged. This charge subsequently leaks away slowly through resistor R2, but may leave the suppressor withconsiderably more negative bias at the time of arrival of the next firing pulse than is provided byits negative bias source alone. The

'suppressoris therefore to some extent automatically biased and so the actual value of negative bias voltage applied is not very critical.

During the meta-stable regime the anode potential of pentode VI will make a linear sweep downward, while its control grid potential makes a substantially linear sweep upward across a small portion or" its, grid base. The velocities of these potential sweeps are determined by the values, of resistor R3 and. condenser CI. The discharging rate of condenser Ci will approximately correspond to the discharging rate which it would have if connected in series with a resistance of value G times R3; across a direct voltage source of value Ghtimes the voltage of the positive line. G being the gain of pentode VI as an amplifier in the present circuit with pentode V2 out 01?. While these sweeps of the anode and control grid. potentials of pentode VI proceed, the

potential. of its screen will remain approximate- I 1y constant; and therefore that of its suppressor also, because the time constant CZRZ is long relative to the duration. of the sweep.

The terminationof the meta-stable regime will occur automatically when the anode of pentode VI hasv reached. a very low potential; because its anode current rapidly falls, and so its screen current increases, with the result that screen and suppressor potentials both fall. The anode current of pentode VI is then cut OE, and the potential drop across resistor RI2 vanishes. Pentode V2 immediately becomes highly conductive; and condenser CI is rapidly charged through resistor RI I, the anode-cathode path of pentode V2 and diode D2. The potential of the control grid of pentode VI rises to earth potential, and the resulting increase of screenv current brings. screen and suppressor potentials down further. Diode D2 prevents the potential of the control grid of pentode VI from rising above earth potential. When they anode potential of pentode VI reaches that. of the slider on potentiometer RI, the stable condition is again established and the circuit is again ready for operation, in response to a. firing pulse. 'It will be evident that the potential selected by the slider on potentiometer RI sets the amplitude of. the potential sweep of the anode of pentode VI, independently, of the velocity thereof.

If the firing is effected by apositive pulse ap 4 plied to the screen of pentode VI from terminal TI, the transition from the stable to the metastable condition will proceed, as soon as diode D3 has become conductive, as the result ofthe same cumulative effects as have been described for a negative firing pulse applied to the anode of pentode VI from terminal T3.

If the slider of potentiometer R14 is adjuste 7 so that the potential to which the anode of diode comes further charged by the current which flows through diode D3 in response to an adequate firing pulse; and this additional, charge has the efiect of biassing the anode of diode D3, for a period after firing, still further than the potential of the slider of'potentiometer RM. This additional bias gradually falls away as condenser C3 discharges through potentiometer RM, butwhile it subsists it establishes the condition that a firing pulse of still greater amplitude would be necessary to re-fire the circuit.

Diodes DI and D3 serve, during the meta--' stable regime, to isolate the generator circuit under consideration from the circuits (not shown) connected to terminals 'It and TI. which; provide the firing pulses.

If it is desired to obtain linear potential, sweeps as output from the circuit, these may be taken from terminal T5 connected t the cathode of pentode V2. If a square-wave of potential is de: sired, this. may be taken from terminal T4 connected to the screen of pentode VI. The form of. the wave front of the square-wave is identical with that, of the upper part of a positive firing pulse applied at terminal TI, provided that this pulse rises rapidly: The screen oi valve VI is at a low potential during the cut ofi period. of this followingmanner. When the anode of pentode VI has reached a very low potential, its anode current falls off, with the result that the potential drop across resistor RI2 falls off, pentode V2 begins to conduct, and the potential of the anode of pentode V2 falls. By virtue of. the coupling of this anode through. condenser C2 to the sup pressor of pentodeVI, the potential of this suppressor is lowered and the. anode current of pentode VI. is further reduced. This effect iscumulative, and anode current through pentode VI." is therefore-completely cut off very quickly.

. Condenser CI now recharges rapidly through the anode-cathode pathef pentode V2 as-described for switch position I. As soon. as this charging. isv completed, the potential of. the anode of pentode V2 rises again, and therefore. likewise that of the suppressor of pentode VI; andanode current begins-to-flow in pentode V-I This flow of anode current leads to the initiation of the metastable regime again, and a further sweep begins Without the need for a firing pulse.

Automatic bias of the suppressor of pentode VI does not occur with the switches in position 2, because the rise of potential of the screen of pentode Vi does not lead to a similar rise of suppressor potential, with consequent heavy suppressor grid current or current through diode D4.

It will therefore be een that with the switches SI and S2 both in position 2, a free-running time-base is provided, giving linear potential sweeps at output terminal T5, while a squarewave of potential in synchronism therewith appears at output terminal T4 or on the anode of V2. Synchronization of this time-base may be effected by the application of either positive or negative synchronising pulses. Positive synchronising pulses will be applied through terminal Ti to terminate the meta-stable regime somewhat earlier than it is naturally terminated when the time-base is free-running. Negative synchronising pulses, on the other hand, will be applied through terminal T3 to limit the amplitude of the charging of condenser Cl and therefore to initiate the meta-stable regime somewhat earlier than it is naturally initiated when the time-base is free running.

The following table gives a set of values for the various components of the circuit, suitable for use for generating linear potential sweeps and square-waves of potential having durations of the order of 500 microseconds.

VI=VR. 116 or Mazda AC/SPl V2=VR. 65 or Mazda SP. 41

RI =50 kilohms (potentiometer) R2=l00 kilohms R3=1.4 megohms (maximum) R4=l megohm R5=200 ohms R6=l0 kilohms R1=5 kilohms Rll3=500 ohms RI l=22 kilohms Rl2=50 kilohms RI3=22 kilohms R|4=50 kilohms (potentiometer) C!=0.0005 microfarad C2=0.0005 microfarad C3=0.0l microfarad 04:2 microfarads C5=0.00005 microfarad By appropriate alteration of the values of components, the duration of the generated potential sweeps and square-wave may be substantially shortened, or may be lengthened to the order of several seconds.

I claim:

1. A circuit for generating saw-tooth and square-wave oscillations, each cycle of said sawtooth cscillations including a substantially linear potential sweep and a flyback, said circuit comprising a first thermionic valve having a cathode, an anode, and a control grid, a capacitor connected between said anode and control grid, an impedance element for discharging said capacitor substantially linearly with fall of voltage at said anode during said weeps, and an anode load for said first valve, said anode load comprising a second impedance element in parallel with a circuit including a second thermionic valve in series with a resistor, said resistor being connected between the cathode of said second valve and said anode.

his

2.A circuit for generating saw-tooth and square-wave oscillations, each cycle of said sawtooth oscillations including a substantially linear potential sweep and a fiyback, said circuit comprising a first thermionic valve having a cathode, an anode, and a control grid, a capacitor connected between said anode and control. grid, an impedance element for discharging said capacitor substantially linearly with fall of voltage at said anode during said sweeps, a terminal for the application of negative-going firing pulses to initiate said potential sweep, a connection between said terminal and said anode, and an anode load for said first valve, said anode load comprising a second impedance element in parallel with a circuit including a second thermionic valve in series with a resistor, said resistor being connected between the cathode of said second valve and said anode.

3. A circuit for generating saw-tooth and square-wave oscillations, each cycle of said sawtooth oscillations including a substantially linear potential sweep and a fiyback, said circuit comprising a first thermionic valve having a cathode, an anode, a control grid and a screen grid, a difierentiating network coupling said anode and said control grid, an impedance element connected as anode load to said first valve, a second thermionic valve having a cathode, an anode, a

control grid and a screen grid connected with its anode-cathode path in shunt with said impedance element, a terminal for the application of positive-going firing pulses to initiate said potential sweep, a connection between said terminal and the first-named screen grid, and connections to render said anode-cathode path of high impedance during said sweeps and of low impedance during said flybacks.

4. A circuit for generating saw-tooth and square-wave oscillations, each cycle of said sawtooth oscillations including a substantially linear potential sweep and a fiyback, said circuit comprising a first thermionic valve having a cathode, an anode, a control grid, a screen grid, a suppressor grid and an anode, a difierentiating network coupling said anode and said control grid, an impedance element connected as anode load to said first valve, a second thermionic valve having a cathode, an anode, and a control grid connected with its anode-cathode path in shunt with said impedance element, connections to render said anode-cathode path of high impedance during said sweeps and of low impedance during said fiybacks and a two-position switch for connecting said suppressor grid through a condenser with the first switch position, to said screen grid and, with the second switch position, to the anode of said second valve.

5. A circuit according to claim 4 including a second two-position switch ganged to the firstnamed switch for connecting said suppressor grid to points at negative potentials relatively to the cathode of said first valve, the said point with the said first switch position being more negative than with the said second switch position.

6. A circuit for generating saw-tooth and square wave oscillations, each cycle of said sawtooth oscillations including a substantially linear potential sweep and a flyback, said circuit comprising a first thermionic valve having an anode, a cathode and at least three control electrodes, a first of said control electrodes being nearest said cathode, a second being between said first control electrode and said anode and the third being nearest said anode, a voltage source having positive and negative terminals, a connection from said cathode to the negative terminal of said source, a differentiating networkeornprising two resistors and a capacitor, a first of said resistors being connected between said anode and one ter' minal of said capacitor, the second of said t re-,

sistors being connected between said other terminal of said capacitor and said positive terminal a of said source, a connection from said other terond valve to said positive terminal, said control I electrode of said second valve being connected to said anode of said first valve, and said cathode of said second valve being connected to said one terminal of said capacitor.

. between the second and thirdcontrol electrodes:

" of said first valve.

anode of said second valve.

3. A circuit as claimed in claim 6, and includ ing a further capacitor connected-between said thirdcontrol electrode of said first valve and the BRIAN CLIFFORD "FLEMING-WILLIAMS.

REFERENCES CITED 7 The following references are of record in thefile offthis patent? UNITED s'rA rEs PATENTS Name Date Whiteley Dec. 10;"1946 Number 

